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Adsorptive characterization of a new generation of biporous materials for liquid phase analytical and preparative separations

Friday, 9 May, 2008 - 17:00
Campus: Brussels Humanities, Sciences & Engineering campus
Jeroen Persoons
phd defence

In this work, the adsorptive and diffusive properties of a new generation hierarchical structured adsorbents in the liquid phase is studied. This was done both in the bulk liquid phase in batch adsorption experiments and under flow conditions in high performance liquid chromatography (HPLC). For both experimental approaches, a case study was done in order to verify the results and test the adsorbents in more realistic experimental conditions.

The biporous materials (zeogrid, zeotile-2 and zeotile-4), which form the central theme of this work, were synthesized in the Centrum voor Oppervlakte Chemie en Katalyse (KULeuven) via a template directed stacking of the precursor (nanoslabs). As a result of this synthesis method these adsorbents have two levels of porosity. The first level of porosity, referred to ad “ultra-micropores”, is located in the precursor material (nanoslab) of the final adsorbent. These nanoslabs with dimensions of 1.3 x 4.0 x 4.0 nm³ have a Silicalite-1 framework (MFI type) and thus contain pore fragments as found in the H-ZSM-5 zeolites (0.5 nm). These nanoslab building units can be aggregated in a controlled way with different templates, to form the second level of porosity, this results in three different biporous materials with various mesopore or “super-micropore” dimensions according to the used template.

Also several conventional zeolites were used in this study, mainly as reference. The used zeolites (H-ZSM-5, NaY, H-USY H-Beta and H-Mor) had different physical and structural properties in order to have an as complete as possible range of porous adsorbents to compare with.

In first instance the general adsorption properties (selectivity and capacity) were studied by means of several tracer/solvent mixtures. All combinations of polar versus apolar were tested in these experiments. The polar tracers (methanol and tert-butanol) were preferentially adsorbed by all three (zeogrid, zeotile-2 and zeotile-4) biporous adsorbents from an apolar n-alkane solvent. Adsorption capacities up to 0.4 ml/g were observed. For these mixtures, adsorption isotherms and uptake kinetics experiments were performed in order to determine intra-particle diffusion coefficients. The diffusion rate of a polar tracer molecule in these biporous solids turned out to be at least three orders of magnitude larger than the diffusion coefficient inside an H-ZSM-5 zeolite.

As a case study to confirm these findings, the adsorption and controlled release of ibuprofen was studied on the biporous adsorbents and, for comparison, on five different types of zeolites. These experiments showed that the adsorption of ibuprofen was not only due to polar interactions, but also apolar. This was concluded from desorption experiments of ibuprofen in a polar solvent (simulated body fluid). On the zeolites, the adsorption as well as desorption did strongly depend on the adsorbent polarity i.e. the Si/Al ratio and the pore size.

The use of these biporous materials as stationary phase in HPLC started with the search for the most suitable biporous solids. After preliminary tests and the development of a modified synthesis method for zeotile-4, a zeotile-4 material with spherical particles was demonstrated to be the most promising. This adsorbent was slurry packed in the column and used as stationary phase. Before the actual experiments were performed, the dead volume of the system and the column were accurately determined with a non-retainable tracer. For a standard mixture of benzene/benzaldehyde/benzyl alcohol, adsorption isotherms were establish on the zeotile-4 column and on a comparable (commercially available) Superspher silica stationary phase, column. These adsorption isotherms again pointed out the very polar (hydrophilic) nature of the biporous adsorbent, since the amount of benzyl alcohol adsorbed was almost the double of that on the Superspher material.

To compare the column performance of the zeotile-4 material to the commercial Superspher adsorbent, van Deemter curves were made and fitted to the reduced van Deemter equation. These experiments showed that the intrinsic performance of this new biporous solid was comparable to or even better than (depending on the tracer) the performance of the self packed Superspher column. In order to obtain this result, a relatively simple procedure was necessary to reduce the particle size distribution, consisting of a fractionation of the zeotile-4 particles by means of sedimentation.

As a case study of the HPLC part of this work, the separation of bio-molecules was investigated in the hydrophilic interaction liquid chromatography (HILIC) mode. The high selectivity for polar molecules together with the promising HPLC characteristics were combined in this particular separation. The separation of several very similar peptides turned out to be possible after an optimisation of the experimental conditions.